A Nonlinear Electrical Resonator as a Simple Touch-Sensitive Switch with Memory ()
1. Introduction
In digital electronics, the quintessential memory element that can be switched between two states is, of course, the flip-flop. The ubiquitous SR flip-flop, for instance, consists of two crossed NOR (or NAND) gates. When no signal is applied, the state of the flip-flop remains in its previous configuration, and in order to flip it to the other state a brief voltage signal (a TTL pulse) is applied to the respective input.
Here we propose a nonlinear electrical resonator that in some ways acts like a flip-flop. As we show, the switching between its two states is accomplished via either a driver-frequency protocol (FSK modulation), or by bringing a magnet or inductor into the vicinity of the resonator; it can also be switched by capacitive coupling. Once set, the system remembers its state until another switching action is performed. However, unlike a flipflop, the element can be induced to switch from the outside of the circuit. Alternatively, a frequency modulation scheme can be employed for fast switching. Finally, we show the application of this idea by constructing a controllable LED array.
Since the switching action can occur in response to touch (via changing the capacitance) or proximity to a magnet or inductor, this resonator acts like touch-sensitive switch and is perhaps reminiscent of a “touch lamp”. When the metal housing of such a lamp is touched, its effective capacitance is increased. There are then a number of ways to convert capacitance to a digital output [1]. Even the simplest scheme incorporates a number of integrated circuit components: a fixed-amplitude AC voltage driver charges and discharges the housing, and the charging current is increased upon touch; further circuitry senses this enhanced current and switches a flipflop. Our nonlinear resonator, in contrast, does not require any further solid-state electronics to act like a switch; no comparators or flip-flops are needed.
Recently, enormous progress has been made in the field of capacitive coupling and sensing, and this has led to the development of touch-sensitive LCD screens. Here again, controllers and micro-processors are incorporated to compute the location of the touch on the screen [2,3]. The power of an array of nonlinear resonators proposed here (see discussion of the prototype) is that no such microprocessing is necessary—the switching action is intrinsic, relying primarily on the bistability of the nonlinear resonator.
Alternatively, fast switching can be accomplished by driving the system at a constant frequency, and then for a brief time interval (given by the FSK modulation pulse width) toggling to another nearby frequency. We show that the pulse width can be as small as two oscillation periods. In the resonator used here, the shortest switching pulse was 7 μs, but this time can be considerably reduced in principle by lowering the inductance value or employing varactor diodes of lower effective capacitance. There is little doubt that switching speeds could reach into the gigahertz range by scaling component properties and boosting the resonance frequency.
The idea of exploiting such resonance bistability is, of course, not new in general. It has been proposed and implemented in a number of physical systems, such as in optical cavities [4,5], in spin systems [6], and micromechanical oscillators [7,8]. Here we present a simple electronic oscillator that works on a similar principle.
2. The Resonance Circuit
Figure 1 depicts the basic nonlinear oscillator. It is comprised simply of an inductor and a varactor-diode in parallel. The latter is a capacitive element since charge is stored across the depletion layer of the pn-junction. As the width of this depletion layer is voltage-dependent, so is the effective capacitance of the diode. Additionally, the diode also allows current to flow through it (preferentially in one direction) and can be modeled as a resistive element with voltage-dependent resistance. The capacitive and resistive properties of the diode can be viewed from a circuit perspective as acting in parallel [9]. Here we choose radial-lead inductors of L =330 mH. The NTE-618 diodes are characterized by an effective capacitance of about 800 pF at zero bias voltage, as well as a large voltage-sensitivity on capacitance; when reversebiasing the diode this capacitance value decreases. The linear resonance frequency is computed as